nrf51: fix AES key setup racing with hardware encryption

[aidankeefe2022]

Fixes Fenrir finding 12349.

wc_AesEncrypt() holds the crypto hardware mutex around nrf51_aes_encrypt(), but wc_AesSetKey() called nrf51_aes_set_key() without it. In SoftDevice builds this could change the global nRF51AesKey pointer between the encrypt path's setter call and its copy into the ECB record. In direct-hardware builds it could reprogram the shared ECB peripheral during an encrypt. In both cases one AES context could encrypt with another context's key.

Keep keys per context, and program them into hardware only while holding the mutex:

• wc_AesSetKey() now only caches the key in aes->key. It no longer touches the hardware or any shared state. It also rejects a NULL userKey.
• SoftDevice: remove the static nRF51AesKey pointer. The ECB record is filled straight from the key passed to nrf51_aes_encrypt().
• Direct hardware: program the peripheral key inside nrf51_aes_encrypt() (under the mutex), right before nrf_ecb_crypt(), and reset it to zeros afterwards.
• Zero the stack ECB record with ForceZero() on the success and error paths.
• Document in nrf51.h that the nrf51 AES functions are not thread safe, that multithreaded builds need WOLFSSL_CRYPT_HW_MUTEX set to 1, and that they must not be called from interrupt handlers.

[Copilot]

🟡 Changes recommended

The hardware mutex remains disabled by default, leaving multithreaded direct-hardware builds vulnerable to the same key race.

1 open finding

• Disabled hardware mutex causes multithreaded AES key races · New

What changed in this PR

Prevents nRF51 AES contexts from racing over shared hardware key state.

Changes:

• Caches AES keys per context and validates null keys.
• Programs and clears hardware keys under encryption locking.
• Clears SoftDevice ECB records and documents concurrency requirements.

File	Description
wolfssl/​wolfcrypt/​port/​nrf51.h	Documents AES locking constraints.
wolfcrypt/​src/​port/​nrf51.c	Removes shared key state and securely handles per-call keys.
wolfcrypt/​src/​aes.c	Caches keys without programming shared hardware.

🧠 Review effort: Balanced

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[wolfSSL-Fenrir-bot]

Fenrir Automated Review — PR #11710

Scan targets checked: wolfcrypt-src, wolfcrypt-bugs, wolfcrypt-port-bugs, wolfssl-src, wolfssl-bugs
Coverage: 3 of 3 in-scope changed file(s) opened by the reviewer

Fenrir result: Approved ✅

No new issues found in the changed files.

Advisory only — this automated result does not count as a GitHub approval.

Review tier: Lite